There's no doubt that more effort goes into HTCs sense than any other UI. It's not to everyones taste and can be a bit too resource hungry for some. I certainly prefer it to Samsungs offering.
That being said, I run Cyanogemmod 7 on my Desire (which doesn't have Sense) so I use Open Home 6.
It's more akin to 'native' Android but with lots more customisation features. The transitions are great and get my i*** friends questioning me relentlessly as to how it does it. The best thing about openhome of course is that it's truly manufacturer independant so can be run on anything. It's certainly my favourite.

There's no doubt that more effort goes into HTCs sense than any other UI. It's not to everyones taste and can be a bit too resource hungry for some. I certainly prefer it to Samsungs offering.
That being said, I run Cyanogemmod 7 on my Desire (which doesn't have Sense) so I use Open Home 6.
It's more akin to 'native' Android but with lots more customisation features. The transitions are great and get my i*** friends questioning me relentlessly as to how it does it. The best thing about openhome of course is that it's truly manufacturer independant so can be run on anything. It's certainly my favourite.

Haven't used Motoblur.
Sense is great. But it's resource hungry, and I find it's beginning to look samey. No matter how well they keep updating it.
I find Touchwiz rubbish. I don't see the point. It adds little imho.
I have to be honest though - I dislike Samsung's garish icons and menu colours. Whenever I use a Samsung Android phone, the first thing I disable is Touchwiz. On the plus side: TW is lighter than Sense in terms of Ram and cpu.
And it doesn't matter much to me, because SPB Shell 3D is the shell replacement to use. Or vanilla Android.
Maybe I'm getting too old, but more and more often I find myself tiring pretty fast of those clever UI add-ons. I know what shortcuts I want and really need. I cannot be blitzed by 10 homescreens for live widgets that suck battery. It's all very cool to spice up reviews and PR material with hot screenshots; to impress your mates, but I like the phone I use to do what I want it to do without a constant 'look at me, how pretty am I?'
Ironically, Apple figured that one out. It's almost Symbian in that iPhone users know what they get, and you don't get distracted by hours and hours of 'personalisation' options.
Sense on winmo was better in my opinion as it never got in the way, and there was more focus on usability than on just 'more more more'.

Sense is absolutely best one available when compared to other phone manufacturers. I would not say it's best when compared to third party offerings. Open Home is pretty good, if you get it working to your liking.
Big drawback with Sense is that there is no way to turn it off.